Nenad Bićanić

Computational framework for discontinuous modelling of masonry arch bridges

Abstract An investigation into using continuum/discontinuum based techniques to analyse backfilled masonry arch bridges is presented. Three different computational modelling tools, discontinuous deformation analysis (DDA), discrete element based analysis using particle flow code and the non-linear finite element analysis using diana have been utilised for this purpose and comparison are made with an experimental model developed at Edinburgh University. Load dispersal through the backfill in the DDA model is improved by using Voronoi tessellation based techniques and the displacement control loading have been utilised in all cases to trace the structural response beyond the collapse load and to identify the collapse mechanisms. The capability and limitations of the methods are demonstrated and some sensitivity analyses are also presented for the semi-circular arch model.

Coupled Heat and Moisture Transport in Concrete at Elevated Temperatures—Effects of Capillary Pressure and Adsorbed Water

ABSTRACT The importance of capillary pressure and adsorbed water in the behavior of heat and moisture transport in concrete exposed to high temperatures is explored by incorporating their behavior explicitly into a computational model. The inclusion of these two phenomena is realized with a formulation of a modified model, which represents an extension to the significant work of Tenchev et al. Comparative studies were carried out, using a benchmark problem, and it was determined that while the Tenchev formulation underestimated the capacity for fluid transport in the concrete, resulting in an overprediction of pore pressures (which may affect the prediction of mechanical damage and spalling), the inclusion of capillary pressure had little effect on the results. More important was the accurate representation of the free water flux, which has a significant effect on the prediction of vapor content and subsequently pore pressure. It was furthermore found that, while the adsorbed water flux may be minimal when concrete is exposed to high temperature, its presence has a significant effect on the fluid transport behavior and the prediction of pore pressures.

Assessment of damage in continuum structures based on incomplete modal information

Abstract An identification technique based on the characteristic equations for structural dynamic systems is developed to assess damage in continuum structures, where the information about incomplete modal data can be directly utilised. The structural damage model is assumed to be associated with a reduction of a contribution to the element stiffness matrix emanating from a given Gauss point, equivalent to a scalar reduction of the material modulus, which characterises material at the Gauss point level. The incomplete mode shapes for the damaged structure can be expanded into complete mode shapes by using the actual measured modal readings and assuming the unknown modal data for the remaining dimension. A computational technique based on the Gauss–Newton Least Squares technique is developed for the inverse prediction of structural damage, where both the location and the extent of structural damage can be correctly determined using only a limited amount of measurements of incomplete modal data. Finally, numerical examples for continuum structures, restricted here to a plane stress model problem and plate bending model problem, are utilised to demonstrate the effectiveness of the proposed approach.

Viscoplastic Hoffman consistency model for concrete

Abstract A novel viscoplastic model for concrete and its performance under dynamic loading is presented. The model is based on the so-called viscoplastic consistency model, recently proposed by Wang. The model makes use of the Hoffman yield function in its isotropic form and is augmented to include seperate hardening/softening behaviour in tension and compression. An implicit backward Euler integration scheme is adopted for the integration of the rate equations. The performance of the model is illustrated on a number of numerical examples at the material point level and on the structural level with the cylinder split test.

Relationship between brittleness and moisture loss of concrete exposed to high temperatures

The effect of moisture loss at high temperatures on the brittleness of concrete was investigated by conducting three-point bending tests on preheated notched beams. The relationships of moisture loss represented by mass loss with heating temperature and exposure time could be established. Higher heating temperature always led to higher mass loss and lower brittleness. Longer exposure time led to higher mass loss and lower brittleness, but this effect was more significant at the early exposure stage and became insignificant thereafter. When concrete is exposed to high temperatures, the brittleness is reduced. The evaporation of gel water was more closely related to the brittleness.

The influence of w/c ratio, concrete cover thickness and degree of water saturation on the corrosion rate of reinforcing steel in concrete

The mathematical model for the chloride-induced corrosion of reinforcing steel in concrete is applied to analysis of the influence of the concrete quality (w/c ratio), concrete cover thickness and the degree of water saturation on the corrosion current density. The governing equations of electrical potential and oxygen transport through concrete, as well as the boundary conditions for the polarisation on the cathodic part of steel surface are described. The numerical procedure based on the finite element and finite difference method is developed to solve the set of equations.

Adaptive mesh refinement for localised phenomena

Abstract The present work is concerned with the development, testing and use of finite element based methods in conjunction with adaptive mesh refinement procedures for the solution of three types of static and dynamic stress analysis problems. First, an adaptive mesh refinement procedure is introduced and used in static plate bending finite element analysis based on Mindlin-Reissner assumptions to study the edge effects which occur in plates with certain types of boundary conditions. Several issues of finite element mesh dependence and adaptivity in strain localisation problems are then discussed and illustrated. Finally, adaptive finite element methods for the solution of two-dimensional transient dynamic stress analysis problems are developed and tested on some benchmark examples. In all cases cited above there is a need to refine the finite element mesh locally in certain zones either within or at the boundary of the domain under consideration. In the transient dynamic analysis there is a further problem; these zones will move with time.

Detection of multiple active yield conditions for Mohr-Coulomb elasto-plasticity

Abstract Singularity indicators that detect if multiple yield surfaces are active and include isotropic hardening/softening for associated Mohr-Coulomb elasto-plasticity are derived. These indicators identify corner and apex singularity regions of the Mohr-Coulomb surface and can be easily incorporated in computer codes. If multiple yield surfaces are seen to be active, a multivector stress return can be initiated. A novel inverted pyramid for the apex singularity region is proposed. Implications of strain and work hardening hypotheses are discussed. Illustrative examples are included.

Identification of structural damage in buildings using iterative procedure and regularisation method

Purpose – The paper aims to identify both the location and severity of damage in complex framed buildings using limited noisy vibration measurements. The study aims to directly adopt incomplete measured mode shapes in structural damage identification and effectively reduce the influence of measurement errors on predictions of structural damage.Design/methodology/approach – Damage indicators are properly chosen to reflect both the location and severity of damage in framed buildings at element level for braces and at critical point level for beams and columns. Basic equations for an iterative solution procedure are provided to be solved for the chosen damage indicators. The Tikhonov regularisation method incorporating the L‐curve criterion for determining the regularisation parameter is employed to produce stable and robust solutions for damage indicators.Findings – The proposed method can correctly assess the quantification of structural damage at specific locations in complex framed buildings using only l...

ASSESSMENT OF TOUGHNESS OF CONCRETE SUBJECT TO ELEVATED TEMPERATURES FROM A COMPLETE LOAD-DISPLACEMENT CURVE--PART 2: EXPERIMENTAL INVESTIGATIONS

Part I of this paper summarized and derived six energy-based and deformation-based toughness indexes for assessing the concrete toughness, based on a two-portion complete load-displacement relationship comprising parameters such as load, characteristic displacements, and geometric coefficients by considering the effect of self-weight of the beam specimen under three-point bending. In this paper, the effects of heating temperature, exposure time, and curing age on the fracture and toughness of concrete were experimentally investigated by conducting three-point bending tests on a total of 55 notched plain concrete beams that had been preheated at temperatures between 100 deg C and 600 deg C over varied exposure times up to 168 hours and for four ages from 7 days to 90 days. The relationships of these parameters with heating scenarios and curing age were established. Thus, six toughness indexes could be used to quantitatively assess the toughness of concrete in varied heating scenarios and curing ages. A higher heating temperature always led to a larger toughness. A longer exposure time also led to a larger toughness, but such effect was more significant at the early exposure stage under 12 hours and became weaker thereafter. A longer curing age only led to a lower toughness in the first 28 days and did not significantly affect concrete toughness after 90 days.